Reliability and speed of communications in a wireless network is increasingly crucial to serve growing user demands. This necessitates increasing improvements in technology.
Wireless communications systems can be deployed using a single transmit and a single receive antenna. The wireless channel distorts and adds other impairments to the received signal. These include additive noise, interference, time selective, frequency selective and space selective fading. Fading implies that the signal can be at different level at different antennas, or frequency or time. It is therefore important to transmit and or receive multiple replicas of the signal from multiple dimensions in space, frequency or time to increase the overall link reliability. This approach is known as diversity and is an important technique to assure reliable wireless communication over fading channels. Space diversity is obtained by using multiple antennas in the transmitter and/or in the receiver
Typically digital modulation of transmitted data is used. Examples of such modulation schemes include M-ary quadrature amplitude modulation (QAM), M-ary phase shift keying (PSK) etc. Multiple access schemes are also employed to support multiple users. Multiple access schemes include code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division modulation (OFDM) and orthogonal frequency division modulation access (OFDMA). Multiple antenna schemes can be used with any modulation and multiple access scheme. In an OFDM system, the operating frequency band is effectively partitioned into a number of “frequency sub channels”, or frequency bins. Each sub channel is associated with one or more sub carriers upon which data is modulated.
The data to be transmitted (i.e., the information bits) are encoded with a chosen coding scheme to generate coded bits. With multiple transmit antennas, coding includes the space dimension along with time or frequency dimensions and is specific to the number of transmit and receive antennas. The encoding scheme determines the diversity that can be captured, the transmission rate and the decode complexity at the receiver. Though different encoding schemes are available in the art, new encoding schemes which enable simpler decoding at the receiver, have good diversity performance and are capable of being used for different multiple transmitter-receiver antennas combinations are desired.
For example, U.S. Pat. No. 6,185,258 discloses the Alamouti code, which is one such simple encoding arrangement scheme in which symbols are transmitted from two transmit antennas over a set of two time slots or frequency sub-channels, with coding that comprises only simple arithmetic operations, such as negation and conjugation. The Alamouti code achieves full transmit diversity. Full transmit diversity is achieved if the diversity contribution from the transmit antennas is equal to the number of transmit antennas. The Alamouti code is a rate one scheme. i.e., it sends on average of one complex symbol per time slot or frequency bin. Use of Alamouti code that codes across two frequency bins instead of two time slots is also known in the art. The number of receive antennas is not specified in the Alamouti code. The code can be used for any number of receive antennas.
However, the Alamouti patent discloses a method of encoding and transmission using only two antennas. Using an Alamouti code over more than two transmit antennas cannot capture the diversity efficiently. Other prior art for more than two transmit antennas use the Alamouti code as a basic ingredient and constellation pre-coding. However, they fail to achieve maximum transmit diversity, and if they do, they are computationally expensive since they do not offer symbol by symbol decoding.
Hence there is a need for an encoding scheme which can be deployed in a wireless communications system with more than 2 transmit antennas and any number of receive antennas, which achieves both maximum transmit diversity with a rate-1 complex symbol per channel use and also offer low complexity symbol by symbol decoding.